1. Field of the Invention
The present invention relates to an improvement of a high pressure metal vapor discharge lamp such as a metal halide lamp or a high pressure sodium lamp, and more particularly to an improvement of a high pressure metal vapor discharge lamp which contains a starting circuit to facilitate start.
2. Description of the Prior Art
In general, the high pressure metal vapor discharge lamp such as the metal halide lamp or the high pressure sodium lamp has a high starting voltage and it is difficult to start such a discharge lamp with a conventional commercial power supply. Recently, it has been proposed to incorporate a starting circuit in the lamp to generate a high voltage pulse, which is then applied to a light emitting tube together with a power supply voltage to facilitate the start. Such a discharge lamp does not use a large size separate stabilizer having a high secondary non-load voltage, does not incorporate a starter in a stabilizer, or does not use a separate starter externally of the stabilizer and it can be used by merely connecting it to an existing high pressure mercury lamp stabilizer. Accordingly, it is very convenient to use and a demand therefor has been rapidly increasing.
Most popular one of such high pressure metal vapor discharge lamps having self-contained starting circuit is shown in FIG. 1, in which a starting circuit 4 having a serial connection of a thermo-sensitive switch 2 which is closed at a room or normal temperature (when the lamp is off) and open at a high temperature (when the lamp is on) and a resistor 3 is connected in parallel with a light emitting tube 1 having at least a pair of main electrodes and accommodated in an external bulb 5 and connected to an AC power supply 7 through an inductive stabilizer 6. As will be discussed later, the thermo-sensitive switch 2 functions to switch on and off a current flowing in the starting circuit 4 to generate high voltage pulses to start the discharge lamp, and after the discharge lamp has been started, it blocks the starting circuit 4 from the AC power supply 7. Usually, it is a bimetal switch. The resistor 3 connected in series with the thermo-sensitive switch 2 functions to limit the current flowing in the starting circuit 4 to an appropriate magnitude, and once the discharge has occurred across the main electrodes of the light emitting tube 1 by the function of the starting circuit 4, it functions to prevent extinction of the discharge even if the thermo-sensitive switch 2 is again closed. Usually, it is a heat generating element such as an incandescent filament.
In this discharge lamp, when the AC power supply 7 is turned on, the current flows into the starting circuit 4 and the resistor 3 generates a heat. As a result, the thermo-sensitive switch 2 is opened. As a result, the current to the starting circuit 4 is cut off and the resistor 3 stops to generate the heat. Thus, the thermo-sensitive switch 2 is closed and the current again flows into the starting circuit 4 and the thermo-sensitive switch 2 is opened again. In this manner, the thermo-sensitive switch 2 repeats the on-off operation and the current in the starting circuit 4 is turned on and off. As a result, high voltage pulses are generated across the stabilizer 6. Since the high voltage pulses are applied to the main electrodes of the light emitting tube 1 together with the power supply voltage, the light emitting tube 1 is fired. After the discharge lamp has been started, the thermo-sensitive switch 2 is kept open by the heat generated by the light emitting tube 1 and the generation of the high voltage pulses is stopped.
The starting circuit used in this discharge lamp is a simple circuit comprising a serial connection of the thermo-sensitive switch such as bimetal switch and the resistor such as incandescent filament. Since it is economic and can withstand the use in a high temperature environment, it has been widely used as the starting circuit contained in the high pressure metal vapor discharge lamp.
However, the above starting circuit has the following disadvantages.
First, since the thermo-sensitive switch 2 is usually the mechanical bimetal switch, a certain degree of ununiformity and variance in the switching operation is unavoidable. As a result, a timing and a magnitude of the high voltage pulse are not constant and a starting characteristic is necessarily unstable.
Secondary, due to the fact that the timing and the magnitude of the high voltage pulse are not constant, an unduly high voltage pulse may be generated, which adversely affects to the discharge lamp and the starting circuit. Accordingly, an abnormal pulse absorbing circuit must be provided. FIG. 2 shows a waveform of the high voltage pulse generated by the starting circuit shown in FIG. 1. As seen from FIG. 2, the interval of the generation of the high voltage pulses and the magnitude of the high voltage pulse are unstable. As to the magnitude of the pulses, a high voltage pulse of 5 KV which is larger than 3-3.5 KV required to start the discharge lamp is generated.
Thirdly, since the resistor 3 of the starting circuit is usually the incandescent filament, the incorporation thereof into the discharge lamp and the anti-vibration means therefor need special techniques.
In order to resolve the above disadvantages, it has been proposed to replace the component of the starting circuit, for example, the thermo-sensitive switch with a voltage-sensitive semiconductor switching device which has no mechanical switching element so that the components of the starting circuit are constructed by semiconductor devices or electronic devices as much as possible.
However, when such semiconductor devices or electronic devices are to be incorporated in the high pressure metal vapor discharge lamp, significant technical problems are encountered. First, since the high pressure metal vapor discharge lamp reaches to a very high temperature during the discharge as opposed to a low pressure metal vapor discharge lamp such as a fluorescent lamp (for example, a temperature in an external bulb of a 400 W high pressure sodium lamp mounted in a lamp case reaches to approximately 400.degree. C. during the discharge), the components incorporated in the discharge lamp cannot maintain the intended functions and are degradated and made unusable in a short period.
In order to start the high pressure metal vapor discharge lamp, a high voltage pulse which is higher in magnitude and larger in energy than that required to start the fluorescent lamp is necessary. When the components which can generate such high voltage pulses under a normal condition and meet the temperature requirement described above are to be used, the sizes of the components necessarily increase and the incorporation thereof into the discharge lamp is difficult to attain.
By those reasons, the high pressure metal vapor discharge lamp which contains the starting circuit constructed by the semiconductor devices or the electronic devices has not been put into practice although it may be good from a theoretical standpoint.